What platform do you run? I am on Linux x86_64 and Fx4b7 takes almost 10% of my RAM and ~20-30% CPU with two tabs open. With the same tabs open in Chromium, I get 2.7% RAM usage and 0-5% CPU usage. Maybe Fx has been really optimizing for Windows lately, but the performance on Linux x86_64 is not very satisfactory.
I am on OS X (10.6.5). I did not see FF4 CPU usage anything near what you are seeing. My CPU is ~2-3% with 6 tabs open (2 being Gmail). RAM usage is steady after a day of using the same browser process.
